"After I pushed him away, he said, 'I’m making sure they’re real,' " she said in an interview with Time.

Maurice (l) and Paul (r) Marciano, CEO and COO of Guess Jeans, in their Los Angeles.(Photo: ROBERT HANASHIRO/USAT)

Marciano has "adamantly" denied the allegations, Guess says.

In its statement, the company says it "takes seriously any allegation of sexual harassment," and "does not condone such misconduct in any form." Guess promised "an extensive and impartial investigation into all of the facts."

The investigation is being conducted by the law firm of O’Melveny & Myers LLP. The board also retained the law firm of Glaser Weil, LLP to advise the special committee.

Upton's allegations have been backed by photographer Yu Tsai who said he witnessed the harassment Upton described.

When Upton rejected Marciano, she says she was fired and labeled "fat." She says he also called her "disgusting" and spread lies about her being drunk or unprofessional on set.

Later, after Marciano hired her again, she says he wrote her "dominant and possessive" messages, became furious that Upton wanted to bring her boyfriend to the Guess shoot, and fired Tsai, a move that seemed like retaliation after the photographer wouldn't let Marciano stay in a room alone with Upton.

Paul Marciano and his brother Maurice co-founded the brand in 1981, redefining the denim business, which is sold in roughly 100 countries.